Pollination Adaptations

Flowers need to be pollinated. Pollination is the process of moving the pollen grain from the anther of a stamen to the stigma of a carpel. There are a few flowers that can self-pollinate all on their own, but this limits them to inbreeding. Most species rely upon some kind of pollination vector to accomplish pollination. The vector can be any agent that can move pollen from anther to stigma.

There is evidence of water and wind as the pollination vector in certain species, but many species do not depend upon the random or downstream-only pollination pathways offered by these vectors. Indeed such vectors are only useful in situations where large populations of a very limited number of species are present.

Most flowers have evolved to use a "smart bomb" or "magic bullet" vector...animals! These vectors have sensory organs to locate flowers, they have locomotion to get them to the flowers in spite of large spaces between individuals, and they have enough intelligence to remember that they can depend upon a reward if they visit one particular species repeatedly.

Attracting the Pollinator

In order to effectively use an animal pollination vector, a flower needs to attract the animal for the first visit. This can be done in one of two ways: visual cues and olfactory cues.

Visual Cues

Some species use a color pattern known as a "bull's eye" to mark their position in the environment...to stand out against a background of green foliage. Obviously not too many animal-pollinated flowers are leaf-green!

Below is a black-eyed susan...recall that there are over 100 species of unrelated black-eyed susans...one of the problems of common names. This one is in the genus, Rudbeckia. Notice its obvious bull's eye target appearance. The bull's eye is black and the yellow ring surrounds it to make it quite noticeable.

The daylily (Hemerocallis) below also shows a prominent bull's eye, but notice how the color pattern is reversed. The bull's eye is light in color and the ring around it is very dark. The effect is the same however, the flower is going to be obvious to a potential pollinator flying overhead.

It is important that whatever color pattern is used is within the visible range of colors observable by the pollinator. Colors obvious to one animal may be invisible to another. The plant shown below is called Gaillardia (painted daisy). This species obviously uses a yellow bull's eye with surrounding red and yellow rings. One gets the appearance of a dartboard just looking at it. The pollinator observed visiting is butterflies. Butterflies definitely are attracted to red/yellow patterns, and they need the large landing platform provided by this wide flower.

On the other hand, Gaillardia does not limit its pollination to just butterflies. An afternoon of observation will tell you that even more visits are made by honeybees! Bees have no ability to perceive red colors, so how does that red/yellow pattern look to a bee? This requires some mental translation and some special photography for a human to appreciate what a bee might observe.

First we need to eliminate red from the image. This can only be accomplished with fancy video work or black-and-white photography. Here are two painted daisies shown in a typical black-and-white photo:

Notice how the red/yellow pattern is now shown in dark/light pattern. This is a black-and-white view of the flower in human vision.

Below is a photo of the same two daisies taken seconds later with a red filter over the camera lens:

Notice how this red filter has removed much of the dark pattern caused by the red pigment. The photograph above thus shows what the flowers look like in the rest of the human visual spectrum (orange, yellow, green, blue, violet). Basically most everything is reflecting these colors of light.

Bee vision does not include red, but it does extend into the ultraviolet. In other words, bees see colors that are invisible to us! Our film is not very sensitive to ultraviolet because it has been developed for showing images visible to humans. To correct that and see the ultraviolet patterns in these flowers, we need to use yet another filter. This filter blocks visible light and allows ultraviolet light to expose the film. Below is the ultraviolet image of the same two daisies taken seconds later with both filters in place:

Notice how the center of each daisy is totally absorbing the UV light. The petals around the perimeter are reflecting the UV light. Thus the bull's eye reflects all colors of bee vision except UV. Since UV is the bee-equivalent of human-purple, the bull's eye can be thought of as appearing as a deep bee-yellow...almost bee-orange. The perimeter ring reflects all colors of bee vision, including UV, so the ring appears as bee-white.

Thus, the red/yellow daisy seen by the human and butterfly appears as a white/yellow daisy to a bee. The flower can thus use either of two pollinators: butterflies or bees.

Another way to attract pollinators is with nectar guides as in the wild carnation shown below:

Nectar guides are color patterns that radiate out from the source of the nectar reward. Like paint on an airport runway, they essentially guide the pollinator to the reward.

Olfactory Cues

Some pollinators are more olfactory than visual, and of course, some use both senses. Flowers have evolved fragrances and this results in efficient pollinator attraction. Again, different animals have different sensibilities and sensitivities to fragrances.

There is a plant called Sauromatum or Voodoo lily which has a stalk of flowers wrapped in a purplish-black leaf. It has evolved to be pollinated by flies. Surely the blackish color of the leaf reminds one of rotting flesh or dung (food and ovipositing objects). The fragrance emitted by this apparatus is very strong...to humans a cow barn comes to mind. Flies are very excited about this fragrance but it is not appealing (or even repulsive) to many other species.

Humans are not as sensitive to fragrances as many other animals, and we certainly have not developed very good terminology to describe fragrances. Nevertheless we appreciate some fragrances such as geraniol from geraniums and roses. Butterflies and birds are not very olfactory. They are much more visual in behavior. Bees are attracted to certain scents, particularly those we might describe as sweet or spicy. Moths and bats are very olfactory and not too visual in orientation. Bat-pollinated flowers usually produce strong fruity or musky scents while moth-pollinated flowers produce very heady sweet fragrances.

Rewarding the Pollinator

Once the pollinator has made the first landing, the flower needs to reward the animal so that it will perceive the reward as a result of its visit. Its intelligence will then allow it to decide to visit similar flowers nearby to obtain additional rewards...carrying pollen from plant to plant in the process. The reward can be nectar, pollen, behavior, or some combination of these.

Nectar Rewards

Nectar is basically exudate from the phloem. It is produced in structures in the flowers called nectaries. The nectary can be on any flower part, and is simply some epidermal area composed of many permanently-open stomata. Beneath the nectary epidermis are vein endings that continually unload sugar from the phloem. Water follows by osmosis and the result is a "bleeding" of sugary liquid through the stomata. The nectar may accumulate in the base of the flower, or perhaps even in a long pouch called a nectar spur:

The violets above show a blue/yellow pattern with nectar guides. The hole in the "bull's eye" connects to a pouch in one of the petals...the nectar spur. This is visible in the flower that is side-ways (on the left) in the photo above. Behind the main petals you can see the pointed spur. This spur contains the droplet of nectar. The bee must siphon the droplet from the bottom of the spur by inserting its proboscis into the center of the flower and down to the bottom of the nectar spur.

Here are some columbine Aquilegia flowers:

You probably noticed that columbine flowers have the red/yellow color pattern to attract butterflies or birds. Further, you might notice that the flower tilts downward. The pollinator has to be below the flower and point its proboscis upward to insert it in the very long nectar spurs. Obviously butterflies do not hover this way, but hummingbirds DO! This nodding flower position with long nectar path is common to hummingbird flowers.

How long is the longest nectar spur? Well, there is an orchid discovered by Darwin that has an 11-inch spur! Darwin hypothesized that there was some moth (the flower is fragrant and has white flowers) with an 11-inch proboscis that would be the co-evolved pollinator. Frustrated, he never found the moth. Fifty years later, the moth was found, its proboscis is indeed 11-inches long, and it does pollinate the orchid!

Pollen Rewards

Flowers, particularly beetle-pollinated flowers need to provide extra pollen for the food needs of pollinators. Bees feed on pollen also. While nectar is mostly sugar and very high-energy food for bees and hummingbirds, pollen is a source of protein, vitamins, and minerals. Fortunately for plants, pollinators feed on pollen but are not fastidious in their own grooming, so they end up depositing at least a few pollen grains on the stigma. One look into a poppy flower shows the vast supply of pollen needed to satisfy a beetle!

Behavior Rewards

Sometimes a flower can get away with some minimal rewards...allowing animal behavior to be the reward. These are some lady-slipper orchids (Cypripedium).

Notice the yellow petal that has evolved as a kind of pouch. This pouch traps a fragrance chemical inside that flies find irresistable. The flies land near the opening of the pouch and climb over the edge, falling inside. The aroma makes the flies act intoxicated. They stumble around and so on for some time. They climb up the back of the pouch toward two "windows" where light is streaming into the pouch. On the way out, the pollen sacs are stuck to their backs. They have had such a great time, they visit a second orchid pouch and, in that visit, the pollen sacs from the first visit are removed from the fly by the stigma...pollination achieved!

Perhaps the most bizzare adaptation of flowers to animal behavior is found in the orchid, Ophrys (sorry, no slide of this one...rats!). This orchid looks like a female wasp and even smells like one. The orchid fragrance is collected by females of the wasp species and is used as a sex attractant for males (a phermomone). The orchids bloom first. Then male wasps emerge from pupae. They smell females! They fly toward the source of the fragrance...there they are! They attempt to mate with the "females," but they are just flowers. The pollen sacs are stuck on the male wasp. The frustrated male moves on to "pseudocopulate" with yet other flowers. The pollen sacs are removed and pollination is achieved. Finally the real female wasps emerge, visit the fading orchid flowers to collect the pheromone, and start to "appear" in the fragrance profile of the male wasps. He mates with the real female, it works!, and the wasps reproduce too. If you think about this, the two species are elegantly timed and chemically and structurally related so that neither can reproduce properly without the other. Amazing!

Shape Matters

Other examples where shape matters include the common tropical Hibiscus flower with the best combination of colors to attract birds:

In the front view, you see an alignment bull's eye. The hummingbird must align the red and yellow bull's eye with the dark-red bull's eye in the base of the flower to guide its beak into the reward. The five dark red objects sticking out the furthest are the five stigmas. The yellow objects are the anthers of the many fused stamens.

The side view shows you the approach that the hummingbird must take to get the reward and achieve the pollination. The bird hovers in front of the flower. As it moves in, its head bumps the stigmas. It moves in closer, the head is rubbing the anthers and gets dusted with pollen. The bird retrieves the reward. It moves to the next flower. Its head bumps the stigmas...depositing pollen from the previous flower...then it moves closer getting pollen from the second flower on its head...and gets the reward. This continues so that each nodding flower pollinates the next one by putting pollen in the head feathers of this hovering bird!

This daylily has a similar spatial arrangement. The stigma extends straight out and will hit the pollinator first. The stamens are gravitropic and curve up so that pollen goes on the pollinator after the stigmatic contact. The reward is at the base of the bull's eye.

Below is another example of bird-pollination...in Tillandsia, a bromeliad relative of pineapple.

Notice that the Tillandsia flowers are not nodding, but instead point up! The color pattern is right for birds, but a hummingbird is not going to be able to harvest nectar from this flower. The angle is wrong! Instead perching birds pollinate it. Of course you need perches since perching birds cannot (or do not) hover. The reddish leaves around the purple flowers are very stiff and provide the needed foot-holds for perching birds. The birds harvest the nectar inside the tubular flower and pollinate them just as for hummingbirds. Note the same relative position of the white stigmas and the yellow anthers, but of course this bird is bending its head down from a perching position.

Now on to some butterfly shape problems:

The plant above is a milkweed (Asclepias). It is pollinated by butterflies. You will notice that the flower shown from a landing perspective has very little "landing platform" but does use the red/pink butterfly motif":

For the butterfly to land here, its legs must go between the the cup-shaped petals that each hold a droplet of nectar. Where the legs grasp the flowerstalk between the petals is a bit of tissue (anther) coated with a botanical sort of "super-glue." The butterfly drinks from the cups, but as it flies off, it pulls the pollen sacs from the flower. They are glued to its legs!

The side view, above, shows the greenish pads where the glue and pollen sacs are. When the butterfly lands on the next flower, the pollen sacs come off on the stigma and pollination is achieved.

Our State Flower

In Connecticut, our state flower is the mountain laurel (Kalmia). It has a wonderful pollination mechanism!

Below are some flower buds of laurel at various stages of development:

Notice how the buds are dome-shaped in the middle, with ten pouches around the edges. Inside these buds there are ten stamens. They grow extensively before the flower opens. As the filaments grow, the anthers hit the top of the dome of the closed bud. Continued filament growth guides the anthers between ridges in the dome so that ultimately the anthers end up in the ten pouches. This bending of the filament provides tension/compression forces in the filament. Then the flower bud opens:

The unfolding of the perianth obviously adds further to the tension/compression forces in the filaments with anthers caught in the pockets. If you look closely in the flower near the center, you can see a red ring around the bull's eye. You can see a white style with green stigma coming out of the middle, and the ten filaments arched with the dark anthers caught in pouches. That flower has opened, but has not yet been visited. A pollinator lands in this flower. You can imagine what happens! The potential energy expressed as tension/compression forces in the filaments is converted to kinetic energy when the weight of the pollinator jars the anthers loose from the pouches.

Bang, bang, bang, bang, bang, bang, bang, bang, bang, bang! The pollinator is bashed with ten stamens from all directions and thoroughly dusted from every angle with pollen! When it lands on another just-opened flower, its body hits the protruding stigma first and pollination is achieved.

You might notice that the several other flowers in the photo above have all been visited. How can you tell?

Next time you see some mountain laurel in bloom (May-June), go up and poke your finger into a just-opened flower and see what happens! It's kind of fun, but remember to do your pollinating job as well!

Wind Pollination

As a parting shot, I thought I'd show you an example of a plant pollinated by wind rather than by an animal.

Remember, the wind only carries pollen down-wind, so it is no "magic bullet." Gravity will also work on the pollen so downward pollination is more likely than upward pollination. There is no way to attract wind...so colors and fragrances would be a waste. Rewards like nectar or behavior are certainly irrelevant. You don't need big landing platforms, nectar spurs, pouches, perches, or other devices relating to creature comforts.

In the photo above, the corn field has plants with male flowers at the top of the plant. These are called the tassel. The female flowers are found in "ears" down on the stalk. That would be a downward pollination direction. Note that corn has evolved to grow in large populations so that it is likely a stigma will be downwind from an anther. Of course it is important that we cultivate corn in that way too. If you are only putting a few corn plants in your garden, remember that a single row will only pollinate well if the prevailing wind is down the length of the row. Since you cannot be sure of this, it is better to plant corn in rectangular blocks of several short rows (a miniature version of the field shown above) rather than in one single long row.

Here is a closeup of the male flowers. This is a stalk with several male flowers on it. Each flower has three stamens hanging down. The filaments are very thin and move with the slightest breeze (the better to distribute pollen from the anther sacs). The anthers are large and produce vast quantities of yellow pollen grains. The grains are dry (not sticky) so that they float on the air individually rather than sticking in clusters and falling to the ground.

So much pollen is blown around in corn fields and nearby that your car is covered...literally dusted...with pollen. Thank goodness that corn pollen is apparently not very allergenic!

The stigma of the female flowers is called a silk. Below is an ear of corn (inside the green leaves called husks). Extending out the end of the husks are the yellow stigmas. They are very long, sticky, and feathery...the better to catch any floating pollen grains. There is one stigma for each kernel in the ear of corn! Each stigma must receive a pollen grain for its kernel to develop properly.

Next time you have an ear of corn in front of you, you can think about how many flowers it takes to make one ear of corn. How many silks had to be removed from it? How many pollen grains had to be caught? Amazing!

 

This page © Ross E. Koning 1994.

 

 

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